1. Field of the Invention
The present invention relates to a fuel cell system operable in a low temperature starting mode.
2. Description of the Related Art
Recently, a fuel cell vehicle employing a fuel cell as a drive source is being proposed. In such a fuel cell, a structure of a predetermined number of unit cells being stacked is known. Each unit cells has an anode, a cathode and an electrolytic film interposed between the anode and the cathode. The fuel cell generates electric power by an electrochemical reaction between hydrogen conducted to the anode and oxygen conducted to the cathode, thereby water is produced. Although this water is mainly produced in the cathode while the fuel cell is operated, there are some possibilities that the water in the cathode move to the anode via the electrolytic film interposed between the cathode and the anode.
When stopping a power generation of the fuel cell, the above-explained water and also humidified water are left within gas flow paths of the fuel cell. Under a low temperature condition, if the power generation of the fuel cell is stopped while such remaining water is left as it is, then the remaining water will freeze, and thus, supplying and exhausting operations of the reaction gases (hydrogen and air) will be disturbed. As a result, a low temperature starting characteristic of the fuel cell is lowered.
To solve the above-described problem, Japanese Patent Unexamined Publication JP-A-2003-203665 proposes to perform the purge process operation to one or both of the anode and the cathode when stopping the power generation of the fuel cell.
However, in a case where the fuel cell is exposed to such a low temperature environment lower than the freezing point or nearly equal to the freezing point, after stopping the power generation of the fuel cell until starting the power generation again, water vapor left in the fuel cell system is condensed. If power generation of the fuel cell is carried out under this condition, then there is such a problem that an efficiency of this power generation is lowered, and the electric power is generated under unstable condition.